This invention relates generally to medical leads and more particularly to implantable cardiac leads.
In the context of implantable leads, and particularly in the context of implantable cardiac leads, there is often a need to remove a lead after it has been implanted in a patient""s body for some period of time. In conjunction with lead removal, it is often necessary to apply traction to the lead, in order to pull it free from tissue adhering thereto. It has therefore been recognized for some time that a reinforcement of some type, extending along the lead body would be beneficial, in order to prevent breakage or partial disassembly of the lead during removal. For example, in U.S. Pat. No. 5,231,996 issued to Bardy et al., a variety of reinforcement mechanisms are disclosed, including cords, filaments, braids, and the like.
More recently, in the context of implantable cardiac leads, the use of cabled or stranded conductors in place of the previously more commonly employed coiled conductors has become more popular. These cabled or stranded conductors, such as disclosed in U.S. Pat. No. 5,584,873 issued to Shoberg et al., U.S. Pat. No. 5,760,341 issued to Laske et al. and U.S. Pat. No. 5,246,014 issued to Williams et al. inherently provide an increased tensile strength lead, at least along the segment between the point at which the stranded or cabled conductor is coupled to an electrode and the point at which the conductor is coupled to an electrical connector at the proximal end of the lead. While this new conductor inherently provides a lead of enhanced tensile strength, in most transvenous cardiac pacing leads employing cabled or stranded conductors, the conductor which extends to the distal-most portion of the lead is still a coiled conductor in order to permit passage of a stylet. This distal-most portion of the lead, particularly in the context of leads employing tines or other passive fixation mechanisms, is the portion of the lead to be most likely to be firmly embedded in fibrous tissue. It is therefore desirable that this portion of the lead in particular should be capable of withstanding high tensile forces without breakage.
The lead disclosed in the present application is particularly designed to reduce problems associated with extraction after implant. In order to accomplish this goal, the lead is provided with three structural features, each directed particularly to providing a lead which is easier to extract and less likely to be damaged during the extraction process.
The first feature of the lead is an improved tip-ring assembly, extending from the tip or distal electrode and including the associated ring electrode located proximal thereto. In particular, the tip-ring assembly is adapted for use in conjunction with electrodes employing passive fixation mechanisms such as tines, in which the tip electrode is fixedly mounted with respect to the lead body, rather than advanceable from the lead body as in the context of a screw-in lead. In order to enhance the durability of the lead during the extraction, the tip-ring assembly is fabricated of three molded plastic components, two of which are fabricated of a relatively rigid plastic, harder than that typically employed in the segment between the tip and ring electrode in bipolar leads employing passive fixation mechanisms. The plastic components are configured to provide a mechanical interlock between the tip electrode and the ring electrode when assembled, and are additionally bonded to plastic insulative tubes or coatings covering coiled and/or cabled conductors extending to the tip-ring assembly area.
In particular, the tip-ring assembly includes a tine sleeve having a central lumen into which a proximal extending shank portion of the tip electrode is inserted, a tip-ring spacer component, adapted to be glued to the proximal end of the tine sleeve and a ring-coil spacer component, adapted to be glued to the tip-ring spacer component, and around which a ring electrode may be located. The ring-coil and tip-ring spacer components together define a circumferential groove dimensioned to receive and retain the ring electrode. The distal end of the tip-ring spacer is configured to overlap the proximal end of the electrode shank located within the tine sheath, so that an generally rigid assembly is provided extending from the distal or tip electrode through and including the ring electrode of the lead. The ring electrode is coupled to a stranded or cabled conductor which extends to the proximal end of the lead, which together with the components of the tip-ring assembly provide a first mechanism for transmission of tensile force applied to the proximal end of the lead all the way to the distal or tip electrode.
A second feature of the invention relates to the provision of insulative coatings or tubings covering these strand and/or coiled conductors employed in the lead which have been treated to enhance their bonding performance, so that they may usefully be adhered to molded or extruded plastic components at either end of the lead, further providing for an additional mechanism of transmission of tensile force along the lead body. In this context, the conductor coupled to the tip electrode may be a coiled conductor surrounded by a heat shrink tube of polytetrafloroethelene (PTFE) which has been treated by etching or otherwise to enhance the ability to bond thereto. The distal end of the heat shrink tube may be bonded adhesively to one or more of the tine sleeve, the ring-coil spacer component and the tip-ring spacer component and to the connector assembly at the proximal end of the lead. The heat shrink PTFE tubing in conjunction with the associated coiled conductor and the adhesive bonds at the proximal and distal end of the lead provide a second mechanism for providing enhanced tensile strength extending along the entire length of the lead. The cabled conductor coupled to the ring electrode referred to above may correspondingly be provided with a plastic insulative coating, treated to improve adhesion. For example, the cabled conductor may be provided with a coating of ETFE, modified by plasma coating using silane gas to provide for increased bonding capabilities. The insulative coating on the cabled conductor may likewise be bonded to plastic components located at the proximal and distal ends of the lead, in turn allowing for distribution of tensile forces between the mechanical joints coupling the cabled conductor to the metal electrode and electrical connector components located at the distal and proximal ends of the leads respectively and adhesive bonds between the insulation and associated nearby plastic parts. The insulation may, for example be bonded to the molded parts associated with the tip-ring spacer and the connector assembly and/or to the extruded plastic tubing making up the lead body. By this mechanism, the ability of the cabled conductors to transmit tensile forces from the proximal end of the lead to the distal portion of the lead without damage to the lead is further enhanced. The improved bonding characteristics provided by surface treatment of the isulative coatings and/or tubes also assist in maintaining effective seals against fluid intrusion and migration within the lead body.
A third feature of the lead intended to improve its extraction characteristics is directed specifically to leads of the type employing elongated coil electrodes, for example as in implantable cardioversion and defibrillation leads. In some leads of this type, the coil is molded into the lead body, such as in U.S. Pat. No. 4,161,952 issued to Kinney et al. However, a simpler alternative construction mechanism is t simply mount coil electrodes fabricated of single or multifilar coils around the exterior of an extruded tubular lead body. Such coil electrodes are disclosed in U.S. Pat. No. 4,934,049 issued to Kiekhafer et al., U.S. Pat. No. 5,115,818 issued to Holleman et al. and U.S. Pat. No. 5,676,694 issued to Boser et al, all incorporated herein by reference in their entireties. Experience has shown that discontinuities in lead diameter associated with the proximal and distal ends of such coil electrodes can complicate removal of the lead from its overlying fibrous sheath. This is true whether the removal is accomplished by attempting to remove the fibrous sheath prior to extraction lead or whether the lead is to be simply pulled through the fibrous sheath. According to this feature of the invention, tubing is provided overlying the extruded lead body intermediate the coil electrodes, if there is more than one such electrode and intermediate the proximal end of the most proximal coil electrode and the connector assembly located at the proximal end of the lead. In this fashion, a lead can be provided which is essentially isodiametric along the length of the lead body to the distal end of the distal-most coil electrode, which lead can be fabricated of extruded multi-lumen tubing and which does not require molding the coil electrode into the lead body. Preferably, if a ring electrode is located distal to the distal-most coil electrode, it too is configured to be essentially isodiametric to the electrode coil and to the lead body or other plastic component separating the distal end of the distal-most coiled conductor and the ring electrode. This particular construction mechanism is especially convenient in the context of a lead of the type employing an extruded multi-lumen tubular lead body, such as that described in the above cited patent issued to Shoberg et al. The provision of tubing overlying the extruded tubular lead body also provides for increased protection of the conductors therein without an over-all increase in lead diameter. This use of tubing which is of increased durability and/or greater insulative strength further enhances this benefit of the lead body structure.